Related fields include semiconductor processing, particularly the controlled etching or removal of silicon oxides.
In the fabrication of semiconductor devices, the removal of a silicon oxide (generically, “SiOx”) layer from a substrate is often necessary. The silicon oxide being removed may be the ˜15 Å “native” oxide that grows when bare silicon (Si) is exposed to oxygen. Alternatively, it may be a thermal oxide (e.g., formed by rapid thermal oxidation (RTO)) or a deposited silicon-containing oxide that needs to be patterned (e.g., a hard mask). Other examples may be found throughout the industry.
Plasma treatment with nitrogen trifluoride (NF3) has been widely used to remove Si and SiOx layers. F* radicals from the plasma react with Si to form gaseous SiF4. Some common process gas mixtures are NF3 with ammonia (NH3) and NF3 with hydrogen (H2). NF3 is highly selective to silicon over silicon oxide; that is, it etches Si much faster than it etches SiOx. As a result, when it is used to remove a SiOx layer from directly underlying Si, the etch rate increases sharply when the reactive F* radical species from the NF3 encounter the Si.
If the etch is uniform and the reaction is not halted soon enough, the underlying Si may be thinned more that the device design will tolerate. If the etch is non-uniform (for example, if the SiOx is porous, as is common in native oxides, or if the SiOx layer has non-uniform thickness, composition, or density), the F* radicals rapidly etch pits under the pores or easiest-etched locations while the rest of the SiOx is still being etched/removed. The resulting surface has pits, patches of unetched SiOx residue, or both, resulting in a surface roughness of 7-11 nm or greater.
As electrical, optical, and other silicon devices shrink in scale, they generally become less tolerant of surface roughness and variations in thickness. Therefore, a need exists for a method of removing native and other forms of SiOx from Si with good control of the process endpoint and with a smooth surface on the etched area.